The disclosed embodiments concern the center section of an aircraft equipped with a working ventral fairing. This ventral fairing is an integral part of the primary structure of the central section, thus making it possible to transfer flexural forces from one part of the structure of the central section to another part of said structure.
The disclosed embodiments have applications in the field of aeronautics and, in particular, in the field of producing and assembling the ventral fairing of an aircraft.
An aircraft includes a body, called the fuselage, and wings, forming the wing structure of the aircraft. This wing structure is mounted on the fuselage and attached by means of a central wing box whose main function is to provide a junction between the two opposite wings of the aircraft. The center wing box, located at least partly in the aircraft fuselage, repeats the internal architecture of the wing structure and extends that architecture inside the fuselage to produce all of the structural junctions with the fuselage.
An aircraft has several sections:                the nose cone, which contains the cockpit,        the front section of the aircraft, which is the part between the nose cone and the wing section,        the tail section of the aircraft, which is the back part of the aircraft after the wing section, and        the center section of the aircraft, which is the part of the aircraft located next to the wings.        
An example of an aircraft with its different sections is shown in FIG. 1. In this figure, the nose cone is marked 10, the front section 11, the center section 12 and the tail section 13.
As can be seen in FIG. 1, the center section of the aircraft includes the wing section 14, the part 15 of the fuselage located next to the wing section (between the two wings) and the center wing box, not visible in FIG. 1. This center section 12 also includes a ventral fairing 20, also called the belly or, in English, the belly fairing.
The ventral fairing 20 of an aircraft is an aerodynamic element enveloping the landing gear system, when said gear is pulled up, and the different operating systems of the aircraft (air-conditioning system, hydraulic tanks, etc.) The ventral fairing is a cowl that serves to close the site of the landing gear while ensuring aerodynamic continuity under the fuselage. To ensure this aerodynamic continuity, the ventral fairing generally has an elongated, profiled shape, as shown in FIG. 1.
An example of a ventral fairing is also shown in FIGS. 2 and 3. FIG. 2 shows a profile view of the ventral fairing 20. FIG. 3 shows a schematic view of the central fairing 20 in the center 15 of the fuselage. These figures show that the ventral fairing 20 is partially placed around the center part 15 of the fuselage, under the aircraft. Side openings 21 in the ventral fairing position said fairing around the wing section. The two wings forming the wing section are each attached, via the openings 21 in the ventral fairing 20, to the center wing box. The ventral fairing 20 is therefore an interface between the wing section and the fuselage of the aircraft.
Traditionally, the ventral fairing is an element independent of the fuselage, attached to the fuselage and the wing section. For this, the ventral fairing has a special structure: it is comprised of an envelope 22 that makes it possible to ensure the aerodynamic continuity of the aircraft and a substructure 23 that maintains the shape of the envelope 22.
The envelope 22 is made of panels generally composed of a fiber-reinforced composite material. This envelope is attached to the substructure 23. The substructure 23 absorbs the pressure forces exerted on the panels of the envelope and the forces created by deformation of the aircraft (flexure of the fuselage, thermal deformation, etc.). The substructure 23 of the ventral fairing is generally composed of frames, wing sections, stiffeners and metal rods, as shown in FIG. 2. It can also include some composite elements, to which are added system supports.
The panels forming the envelope of the ventral fairing, particularly when they are made of composite materials, are attached to the substructure of the fairing. They are not attached directly to the wing section or the fuselage. The consequence of this indirect attachment is the following: in flight, the aircraft is deformed due to the effect of aerodynamic forces; this deformation of the aircraft causes some play in the positioning of the panels in relation to the wing section and the fuselage. The fairing must therefore be attached to the fuselage in such a way as to prevent any interference between the fairing and the fuselage that would produce erosion of the fuselage.
To do so, joints are placed between the ventral fairing and the fuselage to absorb the play between these elements. These joints provide, on one hand, aerodynamic continuity on the fuselage and, on the other hand, a seal between the fairing and the wing section, as well as between the fairing and the fuselage to prevent the flow of water. These sealing joints between the skin of the fuselage and the envelope of the fairing also serve to prevent vibration. More precisely, a ventral fairing joint must perform the following functions:                it must ensure the aerodynamic continuity of the fairing/fuselage and fairing/wing section        it must allow relative movement of the fairing panels in relation to the fuselage or the wing section, and/or        it must provide a seal between the fairing and the wing section and between the fairing and the fuselage.        
However, a ventral fairing joint is subjected to various stresses: its rigidity must be adapted to its function; it should not vibrate and must prevent the vibration of panels in the fairing; it must reduce noise by preventing air from passing through the openings between the structural parts; it must resist attacks from the outside, such as UV rays, air, temperature, ice, erosion due to outside air containing sand particles, etc.; it must also resist attacks from the inside, for example from fuel and hydraulic fluids; it should not damage the aircraft, particularly the panels and the fuselage covering.
To do this, there are traditionally several joint shapes. Examples of joints for the ventral fairing are shown in FIG. 4. The type of joint used basically depends on its function. For example, a flat joint 4A can absorb play and provide aerodynamic continuity; a single-lip seal joint 4B is generally used for seals between fairing and fuselage panels and between wing and fairing panels, when there are no large depressions; a double-lip seal joint 4C is used in place of a lip seal when there are large depressions, for example on the upper surface of the wing; a round, hollow seal is used to provide a good seal by crushing the joint. In some cases, several joints can be combined to perform several functions. An example of a round, hollow joint combined with a single-lip seal joint is shown marked 4D in FIG. 4.
It is therefore understood that the fact that the ventral fairing is an independent part mounted on the fuselage of the aircraft, causes problems with play, friction, wear, relative deformation, etc.
There is also a recurrent problem in the field of aeronautics for aeronautic manufacturers, which is weight gain. In effect, aeronautic manufacturers are constantly trying to reduce the weight of the aircraft, particularly by reducing the amount of material necessary to fabricate the structural elements of the aircraft.